1. Field of the Invention
The present invention relates to an MTR encoding method, an MTR decoding method, an MTR encoder, an MTR decoder, and a magnetic recording device.
2. Description of the Related Art
Conventionally, a magnetic recording device such as a hard disk records information by magnetizing a magnetic film of a built-in magnetic recording medium by longitudinal magnetic recording. The longitudinal magnetic recording is a magnetic recording system in which a magnetization direction in the magnetic film is horizontal to the surface of the magnetic recording medium.
As disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-302154, in the longitudinal magnetic recording system, information is encoded by an RLL encoder, which uses a run length limited (RLL) code, and recorded on the magnetic recording medium. Encoded information read from the magnetic recording medium is decoded by an RLL decoder.
The conventional technologies as above have the following problems. Recently, for improvement of information reproduction performance, an MTR encoder and an MTR decoder have been increasingly used instead of the RLL encoder and decoder. The MTR encoder and decoder use a maximum transition run (MTR) code to stabilize signals by limiting the number of sequential on-bits (bit 1) within a fixed number.
Besides, a recording density of the magnetic recording medium such as a hard disk has been increasing. As one of technologies contributing to the increase in recording density, a perpendicular magnetic recording system is highly promising as a recording system replacing the conventional longitudinal magnetic recording system. Whereas the magnetic film of the magnetic recording medium is magnetized in the horizontal direction in the longitudinal magnetic recording system, the magnetic film of the magnetic recording medium is magnetized in the perpendicular direction in the perpendicular magnetic recording system. This makes it possible to further stabilize the magnetization and improve the recording density.
In the perpendicular magnetic recording system, a reproduced signal has characteristics substantially different from those of a signal in the conventional longitudinal magnetic recording system. The longitudinal magnetic recording system uses a reproduced signal not containing a direct-current component, a peak of which appears at a transition point in magnetization per unit magnetization. On the other hand, the perpendicular magnetic recording system uses a signal having a direct-current component. Therefore, in the perpendicular magnetic recording system, the signal is distorted when the direct-current component is cut off due to influence of an amplifier.
Thus, in the perpendicular magnetic recording system, it is important to control the direct-current component of the signal before encoding to suppress the influence of the amplifier. When the direct-current component of the signal is controlled in advance by convolution, it is possible to suppress the influence even if the direct-current component is cut off because of the limitation of the amplifier.
However, when the signal in which the direct-current component is controlled is encoded by the MTR encoder, the control of the direct-current component is collapsed. In the perpendicular magnetic recording system, it is desirable to use the MTR code to stabilize the signal. However, it is difficult to realize both the MTR encoding and the minimization of the influence of the amplifier through the control of the direct-current component of the signal, which is indispensable for the perpendicular magnetic recording system.
Consequently, it is preferable to achieve MTR encoding of a signal to make the signal more stable by satisfying two constraints: a constraint on the number of sequential on-bits and a constraint on a direct-current component of a signal